1. Field of the Invention
This invention is generally related to a method and apparatus for folding paper boxes and more specifically to a method and apparatus for folding a flap with respect to a panel during the manufacture of corrugated cardboard boxes.
2. Description of Related Art
During the manufacture of paper boxes, paper blanks advance along a paper line for diverse folding and gluing operations. The paper blanks comprise "score lines" that divide the blank into sections. During folding operations, the sections are folded about the score lines to produce the sides, top and bottom of a completed box. In one such folding and gluing operation, preglued edge flaps are folded into a partially overlapping relationship over central, adjoining panels of a blank along certain score lines. The blank is pressed to effect glued joints thereby to produce a completed structure in the form of a folded box.
Prior art apparatus for producing such folding boxes includes a conveyor that engages one or more central panels and advances blanks along a paper line seriatim. The paper line typically parallels those score lines that lie between the flaps and adjoining panels. These score lines define the sites of ensuing operations during which the flap is folded about an axis onto the adjacent panel. The objective of these folding operations is to fold the flaps about axes, called "fold lines", that are coextensive with the preexisting respective score lines.
As the blank advances along the paper line, it passes below one or more backing bars aligned with the preexisting score lines. A single, continuous folding belt system engages the leading edge of the flap and a contiguous area along the surface of the flap. Each belt system comprises a relatively wide belt that runs over a series of pulleys mounted in progressively rotated planes, so the plane of the belt turns from a 0.degree., or initial, plane to a 180.degree., or final, plane. As this occurs, the belt folds the flap onto the central panel. Initially the folding belt system coacts with the backing bar to begin the fold. However, the backing bar usually terminates at a point along the paper line intermediate the folding belt section. This allows the folding belt to force the flap against the panel and produces a sharp, oftentimes creased, corner.
Continuous folding belt systems work quite well with thin cardboard or boxes. However, operating problems can result when these folding belt systems are used to fold flaps onto panels of corrugated cardboard blanks. Apparently these problems arise from the construction of the corrugated cardboard itself. As known, corrugated cardboard blanks comprise paper formed into parallel ridges and grooves sandwiched between cardboard faces. Usually the exterior cardboard faces are finished paper, often with surface printing to appear on the outside of the box. As with other blanks, the corrugated paper blanks have a number of score lines that define the sites for various folding operations.
Score lines are located on a blank independently of the internal structure of the corrugated cardboard. It is therefore a characteristic of these blanks that these score lines that parallel the grooves and ridges of the corrugations are located without reference to the position of the grooves and ridges either on a single blank or with respect to successive blanks. A score line can be located at a ridge, a valley or any intermediate location. However, the position of the score line relative to the ridges and valleys determines, in part, the folding characteristics of the corrugated cardboard. If the score line positions vary within a blank and from blank to blank, then the folding characteristics also vary from blank to blank.
This inherent variation in folding characteristics manifests itself during both manual and automatic folding operations on corrugated cardboard boxes. After a blank is folded along a score line parallel to the grooves and ridges, the actual fold line may skew with respect to the score line. When the fold line skews, the flap does not register with the central panel. This is particularly evident in conventional paper box folding machines using single folding belt systems. It becomes difficult to compensate for the variations in the folding characteristics introduced by shifts in the position of the score line relative to the ridges and valleys, particularly on a blank-by-blank basis.
Moreover it is difficult to maintain the belt velocity of the folding belt system, both in speed and direction, constant relative to the speed and direction of the surface of the flap as it travels along the paper line. If relative motion occurs between the folding belt and the surface of the blank, surface scuffing can occur. As the folding belt system usually engages the exterior surface, any such scuffing can mar the finished surface of the carton or any printing on the carton. Any such marring produces an unusable box.